evaluation of soil erodibility factor (k) forloess derived landforms of kechik watershedin golestan province

introduction: globally, soil erosion is a principal degradation process resulting in negative impacts on different soil functions (food and other biomass production, water storing, filtering and transformation, habitat and gene pool, physical and cultural environment for mankind, and source of raw materials) which ultimately causes irreversible effect on the poorly renewable soil resource. determination of the soil erodibility factor (k-factor) is a cumbersome and expensive undertaking in the effort to predict the soil loss rates. soil erodibility (k-value) is a key parameter in erosion prediction and is important for conservation planning in the face of a rising need for protecting the limited land resources. the technique proposed by wischmeier& smith for estimating the soil erodibility factor is among the most important methods in this regard. materials and methods: given the high amounts of silt and lime content in loess soils of eastern parts of golestan province, the purpose of this study was to evaluate the ability of wischmeier& smith index to estimate the soil erodibility of this region. in this study, soil erodibility was obtained by wischmeier’s nomograph and then was compared with the actual values obtained by selecting three plots and then performing physical and chemical tests on these samples. the universal soil loss equation (usle) developed by wischmeier and smith (1978) is the most frequently used empirical soil erosion model worldwide. soil erodibility is one of six factors affecting soil erosion in the usle that reflects the ease with which soil is detached by splash during rainfall, surface flow or both. to check soil erosion,three plots of 15 meters long and three meters wide with a slope of 16 percentwere selected in the next sites of the station. the plots were separated by metal fences to a height of 30 cm,.to measure the soil profile parameters, the sampling was performedin one stage from depth of0-30 cm in the middle of july 2010 and the samples were transported to the laboratory. the erosion wischmeier& smith index (a), as well as those obtained by swat model and two obvious erosion indices of (r) and fournier was carefully evaluate based on the half-hour rainfall intensity. results and discussion: the analysis of soil profile parameters showed that the soilsweremostly silty loamwith 20.29%sand, 66.54% silt and 13.66% clay, with 2% organic matter and 16.6% caco3. the soil aggregate stability expressed as mwdwas about 0.8 mm. overal, 74 rainfallsoccurredin 2010-2011. the minimum and maximum intensities of deposit-producing rainfalls were 2.98 and 73.589 mm h-1, respectively. using the nomograph, wischmeier index was calculated to 0.05-0.092 mg h mj−1 mm−1. the results showed that wischmeier index was 182 times the actual value of erosion obtained from plots and half-hour rainfall intensity and 4.11 times that value while considering fournier index (r); this parameter was also 6 times the value obtained by swat model and half-hour rainfall intensity and 0.35 times that value while considering fournier index. according to the results,there was a negative correlation between clay and soil erodibility factor in usle, so the rates of erosion in loess soils increases with the decrease in the clay content. aggregate stability was affected by organic matter content and there was a negative correlation between aggregate stability and the k-factor. the results showed that the soil mostly contained silt and had a medium texture. this is due to the presence of loess parent materials in the soils of the study area. based on the laboratory results, the actual soil erodibility was0.35 to 182 times smaller than the value estimated by usle nomograph. the results showed that the parameters used in determining soil erodibility index have shortcomings for use in the soils of the study area. therefore, corrections must be done according to soil characteristics or other indicators should be used. the particle size and the amount of lime in the soil are two factors that affect the index. conclusions: the obtained results showed that the erodibility estimated by wischmeier& smith index was higher than the actual measured value. also,wischmeier &smith;’snomographhas been proposed by assessing the erodibility of almost non- calcareous soils with limited amount of silt. while in arid and semiarid loess soils of golestan province, limestone and siltstone have key roles in erodibilityand aggregate stability. on the other hand, the nomograph is based on rainfalls of semi-humid areas of central america that are different from rain characteristics (intensity and duration) of the study area. poor performance of this index in loess soils indicates the need for further research in this field.